set term gif animate
set output "ball.gif"
set parametric
set xrange [0:10000]
set yrange [-10000:0]
r=200 # 小球半径
do for[i=0:100:1]{
	x = 100*i # 定义变量 x 并赋值：球心横坐标
	y = - i*i # 球心纵坐标
	plot x + r*cos(t), y+r*sin(t) w l lw 3 title sprintf(" t = %d s", i )
}
